Remember when you were a kid and you stuck out your tongue at somebody and made the worst kind of face you could? "Watch out," your mother would say, "or your face is going to freeze that way."

You never really believed her, of course. Well, maybe Mom knew what she was talking about.

Although you aren't likely to stumble through life as the monster you mimicked, the way you habitually use your face muscles, says psychiatry professor Leopold Bellak, can affect your features and expression.

"Life experiences," says Bellak, clinical professor of psychiatry at Albert Einstein College of Medicine in New York, "leave their marks on the surface of the skin." They "also influence the contours of the muscles and bone structure beneath.

"The facial configuration," he says, "is the permanent etching of the disposition, moods and attitudes most characteristic of given person -- the habitual outlook.

"The person who has lived an absorbing, creative and worthwhile life will more than likely have this fact stamped favorably" on his or her face.

What is more, says Bellak, who has spent 30 years studying the subject, people can learn to read each other's faces with a little bit of practice.

He has put his findings in a new book, Reading Faces (Holt, Rinehart and Winston; 163 pages, $10.95), written with Samm Sinclair Baker, author or co-author of 27 mostly how-to books, including murder victim Herman Tarnower's The Complete Scarsdale Medical Diet.

While working with Tarnower, Baker one night met Jean Harris, the woman who was later convicted of shooting the diet doctor.

During pre-dinner drinks at Tarnower's home, says Baker, he joined a group of three women, one of them Harris, who were talking. At the time "I didn't know how she related to Tarnower."

She appeared, he says, "rather attractive" with "an intelligent fact. I like being with women."

But, almost ignoring him, Harris told the other women: "I think it would be demeaning for Hy to write a diet book." She also indicated she didn't think much of how-to book writers.

"I didn't think that was very nice," says Baker. So, "I read her face as she was talking." To do so, he mentally divided her face in sections according to Bellak's "zone system."

The left side of her face, he decided, had a "firm, fixed, rock-like expression."

Looking at her right side, "I was amazed at how it was so highly emotional." There was "sadness" and "a hunger."

In the lower half of her face, the mouth was "tight-lipped." In the upper half, her eyes seemed "worried, unhappy, brooding, centered on the self.

"I didn't say to myself, 'Hey, here's a murderer.'" But he did conclude, "Hey, that's a tough combination. I'm going to stay away from her." Which he did, he says, avoiding invitations to Tarnower's whenever Harris was present.

In his latest book, Baker is the writer and Bellak, author of 30 academic books and also clinical professor of psychology at New York University, the scholar. Says Baker of his friend Bellak, who sought his help: "He wrote three trade books that flopped. He writes stuffily like many scientific people." Baker was on the promotional circuit recently while Bellak lectured in Scandinavia.

The authors point out in their book that what they call the "science of physiognomy -- the ancient art of judging character from facial features" -- is quite different from phrenology , the now-discredited theory that bumps on the skull reveal "special talents and character traits."

Face-reading, says Baker, "is psychologically sound. But I have to emphasize, we don't say it's an exact science." Still, "You can learn a lot from it. It's a great help at giving you indications of character. You can read potentialities of character."

Harry Truman's potential a a president might not have been so under-estimated if his face or photos had been studied during his vice presidency. His face, says Baker, shows the underlying "strength and strong will that made him, as some people say, a great president."

Bellak has found that the right half of the face "often appears more pleasant, sensitive, vulnerable or open in expression." The left half "tends more often to reflect the hidden, severe, stern or depressed aspects of the person underneath."

Advances in split-brain research -- how one hemisphere of the brain influences the opposite side of the body -- have led him to conclude that the left side of the face "is likely to show the more basic dispostion." The right side may show our "more controlled or conscious responses, the civilized social mask.

"That would mean that if somebody has a pleasant left half and a hard, angry right half, his original disposition was probably a pleasant one; he may have developed the hard angry aspect only later in life, in response to specific life situations and experiences."

But when we look at one another, says Baker, it's usually the right side that gives us our overall first impressions. That is why dividing the face into the zone system -- right, left, top, bottom -- is so important to get a more accurate reading of the face.

"You might meet somebody with a bland face," says Baker. "When you split the face, you can see what's behind it," which might be wit and intelligence. "We all wear social masks that can given wrong first impressions."

"Today" show co-host Jane Pauley "always tries to look stern" on camera. "I don't understand that. When you split her face, you see a lot of merriment. You find intelligence in the left part."

The muscles in our face -- there are more, say the authors, than in arms or legs combined -- also can give us away.

The large circular muscle around the mouth, the orbicularis oris , "makes it possible to smile, pout or purse one's lips." How it is used "shapes the character of the mouth. It helps us to identify a genial, affable type accustomed to smiling, or the petulant pouter with his jutting lower lip registering chronic dissatisfaction with life, or the tight-lipped, pinched-mouth type, a rigidly controlled character who goes through life with grim determination."

Habitual frowners, they write, develop furrowed brows. If you're repeatedly anxious, muscles around the eye may give you a telltale wide-eyed fearful expression. The corrugator muscle "pulls the eyebrows together, lending a look of skepticism and disapproval." Wrinkling the nose -- as in sniffing -- amy eventually give a look suggesting a "suspicious, untrusting nature."

Crinkles about the eyes -- like those plainly visible on Baker's face -- also suggest someone ready with a smile.

There is evidence, too, that pulling muscles can even change the bone structure to some extent. "Habitual jutting out of the chin in an attitude of defiance and hostility or clenching the teeth may lead to overdevelopment" of certain face muscles, "creating a lumpy, prominent jaw." Baker calls it "a banker's chin."

Baker puts no great sociological value on face-reading. Though if you are having a relationship or doing business with someone, he and Bellak write, "it is certainly helpful to you to recognize the signs of inner fright, insecurity or chronic anxiety."

If you know your fears will eventually be stamped on your face, they add, you may want to confront them to avoid giving negative signals to others.

When Baker first read his own face in a 15-year-old photo, "I was shocked," he says. Along with such traits he expected -- "cooperative, good-humored, friendly" -- he saw a roughness, cynicism, even ruthlessness.

Since then, "I've adopted more humility. I've been married 30 years, and Natalie says in the last two years I've been a nicer guy.

"If I can get people to use their eyes, to really look at each other and read their faces," says Baker, it might spark more "feeling and communication."

And, he says, "It's fun."